The anesthetic gas components and the types of anesthetic gas that are contained in an anesthesia ventilation gas and the respective percentages at which these are contained in the anesthesia ventilation gas fed to the patient and in the gas exhaled by the patient represent essential information for an anesthesiologist in the course of the anesthesia of a patient. Examples of types of anesthetic gas are, e.g., fluranes, such as desflurane, isoflurane or sevoflurane.
A determination of the particular concentrations of respective types of anesthetic gas is performed here according to the state of the art optically on the basis of a determination of the absorption of optical radiation of certain wavelengths.
The goal is a selective determination of the individual percentages and respective concentrations in an anesthesia ventilation gas, which represents a gas mixture. Devices for such a gas analysis according to the state of the art sometimes have a complicated configuration or it is expensive to manufacture them.
A device for identifying and determining the concentrations of different gas components in an anesthesia ventilation gas, in which radiation components of different wavelengths are transmitted to respective different detectors through a gas cuvette and wherein a respective detector and a respective filter associated with the detector are further used for each particular wavelength, is known from DE 101 40 998 C2. Consequently, a so-called filter bank must be built up in order to measure respective absorptions in the anesthesia ventilation gas at different wavelengths. Such a filter bank is at times so complex that the corresponding configuration requires a certain minimum volume.
The configuration of the filter bank mentioned may also be considered to be an arrangement of a plurality of partial cuvettes connected pneumatically in series with a multichannel detector, wherein the cuvette volumes adding up determine the minimum volume. This minimum volume possibly only permits an inaccurate determination of the concentration in case of abrupt changes in the concentrations of the gas components, because the corresponding pneumatic time constant associated therewith may become relatively high.
Other analyzers for absorption analysis utilize, e.g., a filter wheel in front of a detector, which filter wheel filters out a particular wavelength according to the particular position, and the filter wheel must then be rotated to change the wavelength reaching the detector. It is consequently possible hereby to bring about a change in the wavelength as viewed by the detector, in the sense of a scan, over a certain wavelength range. The drawback is, however, that the filter wheel is subject to a certain wear.
The measurement method known, in principle, from the state of the art for determining a concentration of an individual gas component as part of a gas mixture on the basis of at least two wavelengths will now be explained first in general terms for a better understanding by the reader. An optical signal, preferably infrared radiation, of at least one first wavelength and of at least one additional wavelength is radiated into a volume to be monitored for the gas component or into a gas cuvette to be monitored for measuring a concentration of an individual gas component.
The first wavelength is selected to be such that radiation of this first wavelength through the gas component, whose concentration shall be determined, is absorbed. Such an absorbance can be described by the Beer-Lambert law. A received intensity of the radiation of the first wavelength is then detected by means of a detector located behind the gas cuvette. An indicator of the absorbance of the radiation of the first wavelength can then be inferred from the knowledge of the intensity transmitted at the radiation source at the first wavelength as well as of the intensity measured at the detector. The first wavelength is also called the measuring wavelength.
However, since the radiation of the first wavelength is possibly absorbed not only by the gas component itself, but, for example, also by other effects, such as a contamination of the detector, moisture or condensate present in the gas mixture, or other effects, e.g., aging of the radiation source, the additional wavelength is selected, further, to be such that the radiation of the additional wavelength is not absorbed by the gas component but nevertheless on the basis of the other effects. This additional wavelength is also called reference wavelength. A received intensity can then also be detected for the radiation of the additional wavelength by means of an additional detector located behind the gas cuvette.
An indicator of an absorbance by the gas component in the gas mixture can then be inferred by means of the measured or detected intensities at the two different wavelengths. Taking the Beer-Lambert law into account, the concentration of a gas component in the gas mixture can then be inferred from the absorbance, with simultaneous compensation of the other above-mentioned effects.
The principle of measurement described based on the state of the art can then be used if an absorbance by only a certain gas component or by a particular type of anesthetic gas is to be expected at a first wavelength. However, since it may happen that plurality of gas components or a plurality of types of anesthetic gas may be present in the gas mixture or in the anesthesia ventilation gas, absorbance by a first type of anesthetic gas as well as by a second type of anesthetic gas may occur at the first wavelength. An accurate determination of a first concentration of the first type of anesthetic gas and of a second concentration of the second type of anesthetic gas now requires the determination of an absorbance not only at the first wavelength, but, for example, at three different measuring wavelengths within a wavelength range. The aforementioned additional reference wavelength must likewise be used now for an absorbance measurement, because the effects mentioned shall be compensated during such a measurement as well.
Both the first concentration of the first type of anesthetic gas and the second concentration of the second type of anesthetic gas can then be determined on the basis of such four wavelengths for the particular absorbance measurement.
FIG. 1 shows exemplary absorbance coefficients in a range of values between 0 and 1 for wavelengths in the μm range in the presence of the exemplary type of anesthetic gas halothane or of the exemplary type of anesthetic gas enflurane. The respective absorbance curves KV1 for enflurane as well as KV2 for halothane can be recorded for a defined configuration of a gas and for an exemplary partial pressure, in this case a partial pressure of 50 mbar, as well as for a defined temperature during a reference measurement to be performed beforehand and acquired in a data set. For example, three wavelengths λ1, λ2, λ3 are entered here, which may be suitable for a combined measurement of halothane and enflurane. Further, the reference wavelength λR is entered with 10.5 μm, because essentially no absorbance by halothane or enflurane occurs at this wavelength λR.
FIG. 2 shows additional exemplary absorbance curves KV3, KV4, KV5, which were recorded for the respective fluranes desflurane (curve K4), isoflurane (curve K5) or sevoflurane (curve K3) in a gas cuvette within the framework of a reference measurement or measurement example.